1. Field of the Invention
The invention relates to a steering apparatus for a vehicle such as an automobile, and a control method thereof. More particularly, the invention relates to an automatic steering apparatus for a vehicle that automatically steers steered wheels as necessary, as well as a control method of that automatic steering apparatus.
2. Description of the Related Art
An electric power steering unit, such as that disclosed in Japanese Patent Laid-Open Publication No. 11-78949, for example, has been known, which uses drive torque from a motor of the electric power steering unit to provide steering assist when a vehicle is being steered by a driver, and which further automatically steers steered wheels relative to a steering wheel as necessary.
According to the electric power steering unit in the foregoing publication, it is possible to perform steering assist and, when necessary, automatic steering using one motor. It is difficult, however, to prevent reaction torque generated when the electric power steering unit steers the steered wheels, from being transmitted to the steering wheel, as well as prevent the driver from feeling an unpleasant sensation from that reaction torque. Moreover, the driver also feels a sudden change in torque through the steering wheel when the steering mode changes between steering by the driver and automatic steering.
In order to solve the foregoing problems, an automatic steering apparatus has been proposed that is disclosed in Japanese Patent Laid-Open Publication No. 5-77751, for example. This automatic steering apparatus is provided in a steering system between the steering wheel and the steered wheels, and includes a variable transfer ratio unit (i.e., a variable steering angle mechanism) as steer driving means that steers the steered wheels relative to the steering wheel, and a power steering unit as steering assist force generating means provided in the steering system. Further, this automatic steering apparatus automatically steers the steered wheels using the variable transfer ratio unit, and cancels reaction torque generated by the variable transfer ratio unit during automatic steering with a steering assist force from the power steering unit.
However, in this type of automatic steering apparatus, it is difficult to accurately estimate the reaction torque generated by the variable transfer ratio unit during automatic steering. As a result, it is difficult to accurately cancel out that reaction torque with the steering assist force from the power steering unit. Accordingly, it is difficult to effectively prevent the driver from feeling an unpleasant sensation. It is also difficult to effectively prevent the torque felt by the driver through the steering wheel from changing when the steering mode changes between steering by the driver and automatic steering.
It is an object of the invention to effectively minimize a sense of unpleasantness felt by a driver during automatic steering, and effectively reduce a change in torque felt by the driver through the steering wheel when a steering mode changes between steering by the driver and automatic steering, by controlling an assist steering force generating unit so that a torque acting on the steering wheel during automatic steering becomes a target torque during automatic steering.
A first aspect of the invention relates to a semi-steer-by-wire automatic steering apparatus for a vehicle. The semi-steer-by-wire automatic steering apparatus provided with an assist steering unit, which is provided in a steering system between a steering wheel and steered wheels, and which assists with steering of, and which automatically steers, the steered wheels relative to the steering wheel; an assist steering force generating unit, which is provided in the steering system, and which generates an assist steering force that assists with steering the steered wheels; a detector that detects a torque of the steering system; and a controller which controls the assist steering unit and the assist steering force generating unit. Further, the controller calculates a target assist steering amount of the assist steering unit and controls the assist steering unit based on the target assist steering amount. Further, the controller calculates a first target assist steering force to control a torque acting on the steering wheel to a target torque during non-automatic steering based on the torque detected by the detector; calculates a second target assist steering force to control a torque acting on the steering wheel to a target torque during automatic steering based on the target assist steering amount and a running state of the vehicle; calculates a final target assist steering force as a weight sum of the first and second target assist steering forces; controls the assist steering force generating unit based on the final target assist steering force. Further, the controller decreases the weight of the first target assist steering force and increases the weight of the second target assist steering force during automatic steering as compared to during non-automatic steering.
According to the foregoing first aspect of the invention, the first target assist steering force to control the torque acting on the steering wheel to the target torque during non-automatic steering, is calculated based on the detected torque of the steering system. Also, the second target assist steering force to control the torque acting on the steering wheel to the target torque during automatic steering, is calculated based on the target assist steering amount of the assist steering unit and the running state of the vehicle. The final target assist steering force is calculated as the weight sum of the first and second target assist steering forces. The assist steering unit is controlled based on the target assist steering amount and the assist steering force generating unit is controlled based on the final target assist steering force. Further, the weight of the first target assist steering force is decreased and the weight of the second target assist steering force is increased during automatic steering as compared to during non-automatic steering. Accordingly, during non-automatic steering, it is possible to control the torque acting on the steering wheel to a value near the target torque during non-automatic steering, and during automatic steering, it is possible to control the torque acting on the steering wheel to a value near the target torque during automatic steering. Therefore, by setting the target torque during automatic steering appropriately based on the target assist steering amount of the assist steering unit and the running state of the vehicle, it is possible to effectively minimize unpleasantness felt by the driver during automatic steering.
A second aspect of the invention relates to a control method for a semi-steer-by-wire type automatic steering apparatus for a vehicle. The semi-steer-by-wire type automatic steering apparatus has an assist steering unit, which is provided in a steering system between a steering wheel and steered wheels, which assists with steering of, and which automatically steers, the steered wheels relative to the steering wheel, and an assist steering force generating unit, which is provided in the steering system, which generates an assist steering force that assists with steering the steered wheels. The control method of the second aspect of the invention includes the steps of calculating a target assist steering amount of the assist steering unit; detecting a torque of the steering system; calculating a first target assist steering force to control a torque acting on the steering wheel to a target torque during non-automatic steering based on the detected torque; calculating a second target assist steering force to control a torque acting on the steering wheel to a target torque during automatic steering based on the target assist steering amount and a running state of the vehicle; calculating a final target assist steering force as a weight sum of the first and second target assist steering forces; controlling the assist steering unit based on the target assist steering amount; controlling the assist steering force generating unit based on the final target assist steering force; and decreasing the weight of the first target assist steering force and increasing the weight of the second target assist steering force during automatic steering as compared to during non-automatic steering.
According to this method, it is possible to effectively minimize unpleasantness felt by the driver during automatic steering, just as with the first aspect of the invention.